This invention relates generally to micro scale cavity discharge devices, and, more particularly, to generating a plasma for chemical processing applications.
Exhaust gases from combustion processes constitute one of the main sources of harmful atmospheric pollutants. These pollutants include, but are not limited to, nitrogen oxide (NO), sulfur oxide (SO), soot, volatile organic compounds, hydrocarbons (hydrocarbons are compounds that include two types of elements, carbon (C) and hydrogen (H), such as methane (CH4)), carbon oxide (CO), or the like. It is understood by those skilled in the art that nitrogen oxide (NO) includes N2O, NO2, N2O4, and other similar nitrogen-oxygen compounds. Further, it will be understood that sulfur oxide (SO) includes SO2 and other well-known sulfur-oxygen compounds and that carbon oxide (CO) includes CO2, and other similar carbon-oxygen compounds.
The three-way catalytic converter, which effectively eliminates nitrogen oxide, hydrocarbons, and carbon oxide, is the state of the art specifically for a spark-ignition engine. The catalytic converter is suitable for diesel exhaust gases to only a limited degree because only the hydrocarbons and carbon oxide are largely decomposed, but nitrogen oxide cannot be reduced because of the high oxygen (O) content in the exhaust gas. Until now there has not been any type of catalytic converter with a sufficiently high noxious-substance decomposition rate and lifetime for these types of engines.
Electrostatic filters are well known and commonly utilized especially in controlling particulate emissions. Other forms of electrical discharge devices for the same or similar purposes are less well known but available. For example, there are corona and pulsed corona cells, silent barrier discharge devices, and surface discharge devices.
Generally, in electrostatic precipitators, a high voltage is applied to electrodes to produce a corona discharge. Molecules or compounds in the gas flowing through the device are ionized by electrons and ions of the corona discharge to increase the chemical reactivity. It is known that the conversion efficiency of these devices can be increased if beam-like electrons (electrons having energies higher than about 50 electron volts) are used rather than lower energy thermal-like electrons (electrons having energies typically less than one to two electron volts).
Thermal electrons are produced about 50 nanoseconds after a high-voltage pulse is applied and the higher energy beam-like electrons are dissipated as soon as the thermalized electrical discharge takes place. It would be desirable of course to produce beam-like electrons rather than thermal-like electrons to achieve the highest efficiency. Prior art embodiments have a low chemical conversion efficiency, are not compatible with some chemicals, have a short lifetime, are not selective in regards to the particulate size.
Accordingly, it is an object of the present invention to provide a new and improved method and apparatus for plasma treatment of a chemical species using a micro-scale cavity discharge device wherein the device has an improved performance and a longer lifetime.
To achieve the objects and advantages specified above and others, a method and apparatus for treating a chemical species with a concentration of reactants using a plasma is disclosed. In the preferred embodiment, the method includes the steps of providing an array of micro-scale cavity discharge devices capable of sustaining the plasma wherein the array of devices is defined by a ceramic material assembly. In the preferred embodiment, the chemical species flows proximate to the array of micro-scale cavity discharge devices wherein the plasma interacts with at least a portion of the chemical species to modify the concentration of reactants.
Further, in the preferred embodiment, the array of devices is formed into a screen wherein the chemical species is capable of flowing therethrough. Typically, the chemical species includes hyrdrocarbons, carbon oxide (CO), nitrogen oxide (NO), sulfur oxide (SO), or combinations thereof. The chemical species can be that generated by a diesel engine or the like wherein the array of micro-scale cavity discharge devices are positioned within an exhaust system or similar apparatus.